Fix open review comments on MeshIO PR

[Copilot]

Addresses all unresolved review comments on the MeshIO implementation PR.

MeshIO_PLY.hpp

• PLYType enum replaces std::string for PLYProp::type and list_count_type; parse_ply_type() converts header tokens at parse time. ply_type_bytes, read_ply_binary_prop, and read_ply_prop_from_buf now dispatch via switch instead of sequential string comparisons
• Color-type conditions now match only PLYType::UChar / PLYType::UShort; removed signed char/short cases that could produce incorrect casts for negative values
• trim_right (string_view) replaces trim_right_in_place in all three ASCII read loops; the trimmed string_view is passed directly to split, avoiding a redundant cast

CMake charconv probes

• CheckCharconvFP.cmake now appends needed definitions to EDUCE_CORE_CHARCONV_DEFS instead of calling add_compile_definitions
• CMakeLists.txt applies them via target_compile_definitions(core PUBLIC ...), so they propagate automatically to any downstream target via target_link_libraries

# No manual propagation needed anymore
target_link_libraries(foo PRIVATE educelab::core)

Miscellaneous

• Math.hpp: cross() error message → "Inputs must be 3-dimensional"
• String.hpp: comment at predicate-dispatch call site clarifying it routes to the is_invocable_r_v-constrained overload, not back into the variadic template
• README.md: charconv section updated with correct per-type definition names (EDUCE_CORE_NEED_FROM/TO_CHARS_{FLOAT,DOUBLE,LONG_DOUBLE}) and revised cmake example reflecting automatic propagation
• conductor/product.md: trimmed goal (String) Add partition and to_padded_string #4 per suggestion